1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device in which an analog voltage processing section for sampling and processing analog voltage, and a pulse width modulation (PWM) driving section for generating a PWM driving signal on the basis of digital processing are formed on a semiconductor chip.
2. Description of the Related Art
In some ICs in which digital and analog circuits are combined on a single chip, a master clock in the digital circuit, which is common to other digital circuits, generates a timing signal for determining the sampling time of a sampling processing section included in the analog circuit. In such ICs, there is technology to prevent pulse noise occurring in the digital circuit from adversely affecting the sampling processing of the analog circuit (refer to Japanese Patent Laid-Open Publication No. Hei 9-153802). To realize this technology, the reverse timing of the timing signal is set so as to have a predetermined time difference or more, the amount of which is adequately longer than a delay time per gate of the digital circuit with respect to the rising or falling edge of the master clock.
The technology is effective for a noise source that always has a fixed waveform such as the master clock because the time difference between the reverse timing of the timing signal and the rising or falling edge of the master clock can be set regularly. However, a noise source that has irregular duty ratios, although it may also have a constant period, such as a PWM driving signal for PWM driving of a power device, cannot adopt the technology because the timing of the falling edge (or the rising edge) of the PWM driving signal varies.
Current passing through the power device driven for PWM starts and stops abruptly in accordance with the PWM driving signal. Thus, if, for example, the power device and the IC are connected to a common power source, noise enters an analog voltage processing section in the IC through a power line even when the power device is not mounted on the semiconductor integrated circuit device (IC). Since a motor, a solenoid and the like controlled by the PWM driving has relatively large current carrying capacity, the entry of noise into the analog voltage processing section is a serious problem. This is especially the case in equipment that has to use a battery as a common power source such as car-mounted equipment.